Everything: Interest, Quick results, views, The ability to hide from the worst of the weather in the lea of a hill, and of course "rest time" when you come to a down hill.
We live at the bottom of a 1 in 5 valley, so I
prefer to finish on a downer, it means I am almost home. It also looks better, as the neighbours only see me flying down the hill at the end of my runs. (They are usually in bed when I set out!